The glass was hot. To quote the book: "Fuming: This is a method of exposing hot glass to fumes, usually of tin chloride or silver nitrate, to produce a thin interference layer on the glass. This results in an effect of iridescence. In the past it was common practice to place some of the metallic salt in a tin at the mouth of the furnace and to drop a hot piece of glass..into the tin to create fumes. The glass being worked on would then be held into the fumes to create the iridescence. Today's practice is to mix the metalic salts with alcohol or destilled water and put the mixture into a small hand garden spray. The mixture is then sprayed onto the hot glass in an extraction chamber until the required amount of iridescence is achieved. The hot glass is then reheated in the glory hole before being cracked off to be annealed."